1. Field of the Invention
This invention pertains to high speed, high power permanent magnet electric motors or generators and, in particular, to inserts for installation on the outer surface of the motors or generators for decreasing the windage losses and noise.
2. Description of Related Art
U.S. Pat. No. 6,933,645 entitled “Permanent Magnet Rotor and Magnet Cradle” describes a novel permanent magnet rotor construction for a permanent magnet motor or generator. The rotor assembly comprises a non-magnetic cylindrical shaft having an axis of rotation and a generally cylindrical surface. An even number of recessed and overhung slots in the cylindrical surface define an even number of ribs. C-shaped high magnetic permeability lamination stacks surround the ribs. A permanent magnet is positioned in each recessed slot. The magnets have spaced circumferential end edge faces abutting the lamination stacks. The magnets are magnetized with opposite poles at each circumferential end edge face arranged so that the poles abutting any C-shaped lamination stack are of identical polarity. Non-magnetic cradles support the permanent magnets in the recessed slots. The cradles have axial end walls that bear directly upon the ribs to secure the magnets against centrifugal force.
The rotor described in the preceding paragraph has been found to have excellent characteristics and the cradle design permits an easy assembly. Axial rows of C-shaped lamination stacks are installed on the ribs around one axial section of the rotor by sliding them onto the overhung ribs. Layered permanent magnets are then stacked in cradles and the cradles are slid axially between the C-shaped laminations. This process is repeated until the entire rotor is populated with laminations and magnets. Unfortunately, the fact the axial end walls of the cradles must bear directly upon the ribs leaves a gap between adjacent C-shaped laminations resulting in an uneven assembled rotor surface which results in windage losses.